1. Technical Field
The present invention is detected to a process and apparatus for device fabrication in which an image of a pattern is projected onto an energy sensitive material by illuminating a patterned mask with radiation. The radiation transmitted through the mask is then passed through a back focal plane filter, which has an aperture trough which passes radiation with a certain degree of scatter. The radiation that passes through the back focal plane forms the image in the energy sensitive material.
2. Art Background
Methods for projection lithography in which a back focal plane filter used in a lens system interposed between a mask and the energy sensitive material are disclosed in U.S. Pat. No. 5,079,112, which is hereby incorporated by reference. As illustrated in FIG. 1, delineating energy, shown as rays 10, are directed onto a mask 20 which includes scattering regions 30 and less scattering regions 40. Rays are transmitted through both the scattering regions 30 and the less scattering regions 40. Scattering regions are regions in which the scatter angle of the radiation of consequence is above a maximum scatter angle. Less scattering regions are regions in which the scatter angle of the radiation is below a maximum scatter angle. The rays 11 that are transmitted trough the less scattering regions 40 are not significantly scattered. The rays 12 that are transmitted through the scattering regions 30 of the mask 20 are significantly scattered.
In the single lens system depicted in FIG. 1, the rays 11 and 12 transmitted through a lens 50 onto a back focal plane filter 60. The back focal plane filter is equipped with an aperture 70. The rays 11 are transmitted through the aperture 70 and onto certain portions of an energy-definable resist material. An image 90 of the pattern defined by the mask 20 is thereby introduced into the energy-definable resist material. Rays 12 are scattered beyond a critical scattering angle, and do not pass through the aperture 70, but instead are absorbed or otherwise blocked by the non-apertured portion 80 of the back focal plane filter 60. As described in detail in U.S. Pat. No. 5,079,112, the scattered rays 12 are selectively transmitted through the back focal plane filter to form a negative image in the energy-sensitive resist material if the back focal plane filter blocks the transmission of the unscattered rays and has apertures positioned to permit the transmission of the scattered rays therethrough.
In the above-described method, as well as in other methods for projection lithography, the radiation that is transmitted onto the desired regions of the energy-definable resist material scatters randomly in the energy definable resist material and the underlying substrate. This scattered radiation then exposes regions of the energy-definable material that are not intended to be exposed to the patterned radiation. The region of the energy-definable resist material that is adjacent to the exposed region is exposed to more backscattered radiation than the region of the energy-definable resist material that is not adjacent to the exposed region. This causes an unwanted contrast in the unexposed region of the energy-definable resist material. This unwanted contrast is known as the proximity effect and is described in Owen, G., et at., "Proximity effect correction for electron beam lithography by equalization of background dose," J. Appl. Phys. 54(6), pp. 3573-3581 (June 1983), which is hereby incorporated by reference. Owen et al. suggest using a correction exposure to correct for the proximity effect. The correction exposure mimics the distribution of the backscattered radiation in the reverse field of the pattern. This technique requires two exposures, the pattern exposure and the correction exposure. Since it is desirable from a manufacturing standpoint to limit the number of exposures, a method for proximity effect correction that does not require multiple exposures is desired.